Comment # 15 on bug 75127 from Alex Deucher

Please try these patches:
http://cgit.freedesktop.org/~agd5f/linux/commit/?h=drm-fixes-3.14&id=9babd35ad72af631547c7ca294bc2e931cc40e58
http://cgit.freedesktop.org/~agd5f/linux/commit/?h=drm-fixes-3.14&id=7848865914c6a63ead674f0f5604b77df7d3874f

You can also force runpm off by booting with radeon.runpm=0 on the kernel
command line in grub.